1. Field of the Invention
The present invention relates to a 90-degree phase shifter, and more particularly to a 90-degree phase shifter that is built using a T flip-flop.
2. Description of Related Art
An example of the configuration of a conventional 90-degree phase shifter is shown in FIG. 3. The conventional 90-degree phase shifter shown in FIG. 3 is a 90-degree phase shifter built using a T flip-flop, and is composed of transistors Q1 to Q12, resistors R1 to R4, input terminals 1 and 2, constant current sources 3 and 4, a constant voltage source 5, and output terminals 6 to 9.
When an input signal having a predetermine frequency and having a duty factor of 50% is fed in via the input terminal 1, the input transistors Q1 and Q8, of which the bases are connected to the input terminal 1, repeatedly turn on and off according to the input signal. When a signal complementary to the input signal is fed in via the input terminal 2, the input transistors Q2 and Q7, of which the bases are connected to the input terminal 2, repeatedly turn on and off with the timing opposite to that with which the input transistors Q1 and Q8 turn on and off.
As a result, a first frequency-divided signal (0-degreee signal), which is a signal obtained by performing ½ frequency division on the input signal and of which the zero cross points are synchronous with the rising zero cross points of the input signal is fed out via the output terminal 6, and a signal (180-degree signal) complementary to the first frequency-divided signal is fed out via the output terminal 7. Moreover, a second frequency-divided signal (90-degreee signal), which is a signal obtained by performing ½ frequency division on the input signal and of which the zero cross points are synchronous with the trailing zero cross points of the input signal is fed out via the output terminal 8, and a signal (270-degree signal) complementary to the second frequency-divided signal is fed out via the output terminal 9.
When the input signal is free of any DC offset or distortion, the input and output signals behave, for example, as shown in the time chart in FIGS. 4A to 4C. In FIG. 4A, A indicates the input signal that is fed in via the input terminal 1, A-bar (overscored A) indicates the input signal fed in via the input terminal 2. In FIG. 4B, B indicates the output signal fed out via the output terminal 6, and B-bar (overscored B) indicates the output signal fed out via the output terminal 7. In FIG. 4C, C indicates the output signal fed out via the output terminal 8, and C-bar (overscored C) indicates the output signal fed out via the output terminal 9. When the T flip-flop operates in an ideal manner on an ideal input signal like the input signal A, the phase difference between the two output signals (the output signals B and C) is exactly 90 degrees.
On the other hand, if the input signal contains any DC offset and/or distortion, or if the circuit elements that constitute the T flip-flop have variations in their characteristics among them, the phase difference between the two output signals, undesirably, deviates from 90 degrees. For example, if the input signal contains a DC offset, the input and output signals behave, for example, as shown in the time chart in FIGS. 5A to 5C. In FIG. 5A, A′ indicates the input signal that is fed in via the input terminal 1, A′-bar (overscored A′) indicates the input signal fed in via the input terminal 2. In FIG. 5B, B′ indicates the output signal fed out via the output terminal 6, and B′-bar (overscored B′) indicates the output signal fed out via the output terminal 7. In FIG. 5C, C′ indicates the output signal fed out via the output terminal 8, and C′-bar (overscored C′) indicates the output signal fed out via the output terminal 9. Since the input signal A′ contains a DC offset, its duty factor is not exactly 50%, and this deviation causes the phase difference between the two output signals (the output signals B′ and C′) to deviate from 90 degrees.
A 90-degree phase shifter designed to offer a solution to the above problem is proposed in Japanese Patent Application Laid-Open No. H8-237077. The 90-degree phase shifter proposed in this publication is configured as shown in FIG. 6. In FIG. 6, such circuit elements as find their counterparts in FIG. 3 are identified with common reference numerals or symbols.
As compared with the conventional 90-degree phase shifter shown in FIG. 3, the conventional 90-degree phase shifter shown in FIG. 6 is additionally provided with a 90-degree phase comparator 10, a low-pass filter 11, a DC amplifier 12, and capacitors C1 and C2.
In the conventional 90-degree phase shifter shown in FIG. 6, the 90-degree phase comparator 10 detects the phase deviation from 90 degrees. The low-pass filter 11 and the DC amplifier 12 extract, from the output of the 90-degree phase comparator 10, the direct-current component that corresponds to the phase deviation, and then feed it back to the control terminal (base) of each of the input transistors Q1, Q2, Q7, and Q8. This permits a direct-current bias to be applied to the base of each of the input transistors Q1, Q2, Q7, and Q8 in such a way as to correct the phase deviation from 90 degrees, eventually eliminating it.
The conventional 90-degree phase shifter shown in FIG. 6, however, has the following disadvantages. First, since the phase deviation from 90 degrees is fed back as a voltage, how it is fed back tends to be influenced by noise. Second, also since the phase deviation from 90 degrees is fed back as a voltage, how it is fed back tends to be influenced also by the voltage drop across the wiring resistance of the path by way of which it is fed back. Incidentally, since a 90-degree phase shifter is typically built into an integrated circuit, the just mentioned wiring resistance is usually comparatively high.
Hence, under the influence of noise or of the voltage drop across the wiring resistance as described above, the conventional 90-degree phase shifter shown in FIG. 6, inconveniently, does not always yield output signals with a phase difference of exactly 90 degrees.